(i) Field of the Invention
The present invention relates to an insulated-gate field effect transistor (will be termed simply IGFET hereinafter) and a composite semiconductor device made up of IGFETs and the method of fabricating the same.
(ii) Description of the Prior Art
The conventional IGFET has a lateral channel structure, and its basic elements include a source, drain and gate. The source and drain are disposed laterally on the surface of the substrate, and a current flowing between the source and drain is controlled with the gate located between the source and drain.
Fabrication of a large number of IGFETs having uniform characteristics in a wide area (20 cm by 30 cm or more) need the use of chemical vapor deposition, e.g., plasma chemical vapor deposition in which a semiconductor layer is formed at a temperature of up to 500.degree. C. by utilization of glow or arc discharge. However, the semiconductor layer formed by the chemical vapor deposition (CVD) method is non-single crystal semiconductor, providing an extremely low carrier (electron and hole) mobility which is 1/10 to 1/100 of that of monocrystalline semiconductor, and therefore the channel length must be made smaller than 2 .mu.m (preferably 0.1 to 1 .mu.m) so that a complete device has a practical frequency response.
When the CVD method is used in the fabricating process for the conventional lateral channel structure in which the source and drain are disposed laterally, the accuracy of alignment is limited, and therefore it is difficult to make the channel length below 20 to 40 .mu.m over a large area.
Non-single crystal semiconductor formed by the CVD method has an amorphous structure, a semi-amorphous structure having a quantum-theoretic orderly nature or fine crystalline nature of a size of 5 to 100 .ANG., or a micro-polycrystalline structure with crystals in a size of 100 to 2000 .ANG.. However, non-single crystal semiconductor added by hydrogen or halogen such as fluorine has not as high density as monocrystalline semiconductor has. On this account, it is difficult to form a satisfactory pn junction, or to fabricate a semiconductor device with a breakdown voltage above 50 volts.